Glass compositions have been used in various medical and dental prostheses, hard tissue cements, and dental filling compositions. It is highly desirable for a medical or dental prosthesis, hard tissue cement, or dental filling composition to be radiopaque so that it is detectable by X-rays. For example, from radiographs a dentist determines the condition of a dental filling, e.g. whether it has cracked, or whether decay is occurring at the interface between the tooth and the filling. Fillings and restorations which are made of metal are readily observable in X-rays. Fillings of the porcelain/plastic art are not observable by X-rays unless they have radiopaque materials therein.
Currently, dental filling materials are rendered radiopaque by incorporating barium into the inorganic powder moiety of the filling material. The most effective radiopaque agents are elements of high atomic number (i.e. the "heavy elements" of the periodic table); it is unfortunate, however, that most of these elements are either radioactive or toxic, such as thorium or lead. Barium is toxic also, but in certain medical uses it is present in a form so highly insoluble that the body is unable to metabolize enough of it to become intoxicated. In dental applications barium glasses have been used as components of dental restorations, on the hypothesis that barium ions within the structure of a glassy matrix will not be available to oral fluids (saliva, beverages, etc.) and will not, therefore, pose a problem of toxicity. Examples of the use of barium glass in dental restorations can be found in U.S. Pat. Nos. 3,801,344; 3,808,170; 3,826,778; 3,911,581; 3,959,212; 3,975,203 and 4,032,504. Unfortunately, in practice, the barium glasses are not as stable as had originally been hoped, and they have not, therefore, found favor in the art on account of the risk they pose of poisoning the patient (see, e.g. U.S. Pat. No. 3,971,754). A further problem encountered with the barium glasses is that of matching refractive indices to that of the other components of the restoration. For example, it would be desirable to use components with refractive indices in the range of about 1.5 to 1.6 (so as to closely match the refractive index of commonly used organic binders) but most barium glasses with refractive indices in this range are unsuitable for dental use according to U.S. Pat. No. 4,032,504. It is difficult, therefore, to prepare restorations containing barium glass which present an unobtrusive appearance when used for anterior surface repair. An additional problem of the barium glasses is their alkalinity. Typically, barium glasses show alkalinity values of pH 9 or greater, whereas a pH of 7 is preferred. Highly alkaline fillers appear to degrade the siloxane coating resulting from etching of the prepared tooth cavity and also cause rapid decomposition of any peroxide catalyst present in the dental restorative composition during storage.
Recent efforts in the field of dental restoration materials have resulted in the use of fillers other than barium-containing compounds as an X-ray detectable component. For example, U.S. Pat. No. 3,971,754 describes the use of certain oxides or carbonates, particularly those of lanthanum, strontium, tantalum and, less usefully, hafnium. These salts are mixed with glass-making components at the time the glass is made, yielding a lanthanum, strontium, tantalum or hafnium glass which possesses a measure of radiopacity. U.S. Pat. Nos. 3,973,972 and 4,017,454 describe glass ceramics which possess both a low coefficient of thermal expansion (an advantage in dental fillings) and a useful degree of radiopacity, by virtue of a high content of rare earth elements, particularly lanthanum. The rare earth elements absorb X-rays in the wavelength range of 0.2-0.3A, a range commonly available from dental X-ray machines. However, the cost and problems with availability of these rare earth fillers make them generally unsuitable for commercial use.
In another approach to preparing radiopaque composites for dental use, organic halide (e.g. an alkyl iodide) has been incorporated into plastic materials (e.g. acrylate polymers), from which molded articles are made (e.g. U.S. Pat. No. 3,715,331). However, the articles molded from such compositions lack the strength of restorations made from glass or ceramic materials.
U.S. Pat. No. 4,250,277 describes a glass composition used for crosslinking polycarboxylic acid cement, wherein the glass contains zinc oxide and a large amount of boric oxide, in addition to other ingredients. This glass, however, is too water soluble to be useful in dental restorative compositions and prosthetic devices.
U.S. Pat. No. 4,215,033 describes a composite dental material containing a glass which in one embodiment is described as single phase. However, the patent does not appear to recognize that a single phase glass containing zinc oxide can be made radiopaque. Also, the single phase glass composition described in this patent is very difficult to make. Furthermore, such glass does not contain any aluminum fluoride.